The diffuse nebulas are always associated with the gas and dust grains of space, which stay in the spiral arms in the plane of the Milky Way.  In most areas of space, the dust and gas are cold, but may increase in temperature in the presence of a hot star.  The Great Orion Nebula can be seen because there are hot stars in the inside area of the Great Orion Nebula.  The clouds in the Taurus constellation are just as dense, but they do not have any bright or hot stars.  However, the diffuse emission nebulas may be in the process of forming.